Forecasting change of the magnetic field using core surface flows and ensemble Kalman filtering

Accurate forecasting of the change of the Earth's internal magnetic field over short intervals of time (e.g., less than five years) has many applications for government, academic and commercial users. Forecasting can be achieved by making a number of reasonable assumptions about how the main field interacts with the flow in the liquid outer core. In particular, the magnetic field can be considered to be entrained in the large scale flow along the core‐mantle boundary surface over short time periods, giving rise to measurable change at the Earth's surface. The observed change (or secular variation) at or above the surface of the Earth can thus be inverted to produce flow models; these can be used to propagate fluid parcels threaded by the field forwards in time to forecast the non‐linear change of the magnetic field. In addition to prediction of field change by flow models, it would be advantageous to include observations of the field from satellite measurements or ground‐based observatories. We therefore present a method using Ensemble Kalman Filtering (EnKF) to produce an optimal assimilation between magnetic field change as forecast from core flow models and direct observations of the field. We show, by assuming a steady flow and assimilating field observations annually, it is possible to produce a forecast over five years with less than 30nT root mean square difference from the ‘true’ field – within an assumed error budget. The EnKF method also allows sensitivity analysis of the field models to noise and uncertainty within the physical representation

The magnetic field of the Earth's lithosphere

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A mantle magma reservoir beneath an incipient mid-ocean ridge in Afar, Ethiopia

International audienceShallow magma reservoirs exist in the crust beneath volcanoes and mid-ocean ridges, yet there are no reports of extensive magma bodies within the uppermost mantle. Indeed the buoyancy of magma should cause it to intrude into the crust, preventing it from ponding in the mantle below. The Dabbahu magmatic segment in Afar, Ethiopia, marks the late stages of continental rifting. This segment has been active since 2005 and has experienced repeated magma intrusions1, 2, 3, 4, 5, 6. Here we use magnetotelluric data to image magma bodies beneath it. We identify a 30-km-wide region of very high electrical conductivity that reaches down to about 35 km depth. We interpret this region as a large volume of magma of at least 500 km3 that extends well into the mantle and contains about 13% melt fraction. The magma volume is orders of magnitude larger than that intruded during a typical rifting episode, implying that the magma reservoir persists for several tens of thousands of years. This is in marked contrast to the situation beneath mid-ocean ridges, where melt supply is thought to be episodic7, 8, 9, 10, 11. Large magma reservoirs within the mantle may therefore be responsible for the localization of strain that accompanies the final stages of continental break-up